Experiments will continue with isolated coronary arteries of the dog, with as its ultimate goal the understanding of the causes of coronary vasospasm, in particular that resulting from hypoxia. The proposed research will focus on the changes in responsiveness of the large coronary arteries associated with reperfusion of the myocardium after a period of circulatory standstill. The major working hypothesis tested is that the ischemic episode not only endangers the tissue cells when the flow of blood is resumed, but that it also affects, probably permanently, the ability of the endothelial cells to produce relaxing and contracting factors. If this were to result in an imbalance favoring vasoconstriction, such an affect may help to explain the reported episodes of spasm after surgical or chemical reperfusion of the coronary vascular bed. To determine the status of the endothelium, we will compare the responses of rings of coronary arteries with and without endothelium. To measure endothelium-derived relaxing factor(s), we will bioassay the perfusate from segments of coronary arteries with endothelium; the perfusate will be analyzed chemically to detect nitric oxide, amonia and metabolites of arachidonic acid. To determine the release of endothelium-derived contracting factor(s), we will use layered preparations ("sandwiches"). The responses of control coronary arteries will be compared to those of arteries which have been exposed to ischemia (cessation of blood flow), hypoxia (perfusion with hypoxic blood) or either maneuver combined with reperfusion. The responses to be studied include those to substances which release endothelium-derived relaxing factor(s) or cause direct relaxation of the smooth muscle, as well as contractions evoked by hypoxia. Studies will determine whether or not the acute alteration in endothelium-dependent reactivity caused by reperfusion is readily reversible, and whether or not the injury results in chronic changes in responsiveness of the coronary artery. The extent of platelet and leukocyte deposition in reperfused arteries will be measured. Several hemodynamic maneuvers and pharmacological agents will be tested for their ability to attenuate the consequences of reperfusion at the level of the large coronary arteries. The studies will improve the understanding of the reperfusion injury observed in patients when flow is reinstated after a period of ischemia, and may help to find ways to prevent or attenuate this process.